Ideally, a blade of the airfoil type is shaped like a typical aeroplane wing, where the chord plane width of the blade as well as the first derivative thereof increase continuously with decreasing distance from the hub. This results in the blade, ideally being comparatively wide in the vicinity of the hub. This again results in problems when having to mount the blade to the hub, and, moreover, this causes great loads when the blade is mounted, such as storm loads, due to the large surface area of the blade.
Therefore, over the years, the construction of blades has developed towards a shape, where the blade consists of a root area closest to the hub, an airfoil area furthest away from the hub and a transition area between the root area and the airfoil area. The airfoil area has an ideal or almost ideal blade shape, whereas the root area has a substantially circular cross-section, which reduces the storm loads and makes it easier and more safe to mount the blade to the hub. The root area diameter is preferably constant along the entire root area. Due to the circular cross-section, the root area does not contribute to the production of the wind turbine and, in fact, lowers the production a little because of wind resistance. As is suggested by the name, the transition area has a shape gradually changing from the circular shape of the root area to the airfoil profile of the airfoil area. Typically, the width of the transition area increases substantially linearly with increasing distance from the hub.
It is well known from the aeroplane industry that aeroplanes built with two wings, so called biplanes, can normally lift more than an aeroplane with only one wing. This allows for an increase of the total lift of the wings of the aeroplane without increasing the width of the wings.
This principle is also know in connection with blades for wind turbines, i.a. by manufacturing wind turbines with two or more rotors. CA 2 395 612 describes a wind turbine with two co-axial rotors where one rotor rotates faster than the second. GB 758 628 describes a wind turbine with two co-axial rotors rotating in opposite directions.
WO 98/31934 discloses a blade designed like with a biplane. The blade is constructed by means of two parallel spars mutually connected by means of cross braces. Two streamlined elements are fixed on each spar, said elements comprising a leading edge and a trailing edges, respectively, together providing an airfoil.
U.S. Pat. No. 5,161,952 discloses a biplane construction for wind turbine rotors. The wind turbine is designed so that two straight blades are connected with the hub of the rotor at a distance from each other. The tips of the two blades are interconnected.